This is my document recording my experience in upgrading a G36C V3 gearbox for CQB.

The beginning
I began with a used pre-2gx G36C purchased on craigslist from a local. It is one with the jumbo handguard. It had been rebuilt at the KWA shop and had the "do not remove" sticker on it.
Chrono'd at a very consistent 360FPS 17 ROF with a 9.6v. Shot very straight. This G36C would have been just fine as is and may have lasted a long long time. Hindsight is priceless.

The Goal
Originally I wanted a higher ROF and should have just stuck an 11.1v lipo in and let her rip. But no I had to open it up, adjust AOE and put a stronger spring in to counter possible overspin as several posts here recommended. I put some sorbo on the CH and ground the second tooth from the pickup tooth from the piston and put it together with a FET and Deans. It stripped the piston after about 100 cycles. An expensive lesson.

Now my new goal for the rebuild would become ~25 RPS with somewhere around 330- 350 FPS. The field I frequent is indoor where the long shots are less than 120'. The field has a 400FPS limit for full auto and 410 for semi only. Personally I don't want to hurt anyone and use .20's because most of my engagements are under 50 foot. For more on bb weight vs engagement distance take a look at the Airsoft Trajectory Project herehttp://mackila.com/airsoft/ATP/

Another goal is to spend the least amount necessary to get this gun fieldable again.

Efficacy
Jumping ahead to the end of my July 2012 build here is what the result was.360-411 FPS depending on the hop-up position. Adjusting the hop up for a flat trip out to 120' the FPS is 391 or so. Very stable FPS once the hop-up is set.Rate of Fire is 23.75 rounds per second.
Feeds beautifully, shoots straight, hop up very effective.
This gearbox will not cycle with an 8.4VDC NiMH battery

Process

What was done to the gun:
New piston to replace the old ruined one
Lighten piston to allow it to cycle faster
Short stroke piston to allow it to cycle faster
Remove second tooth from pickup side of piston and half of 3rd tooth to correct AOE and preclude PE
Remove rubber pad from CH and install sorbo and put rubber pad on top of sorbo to correct for AOE
Glue the cylinder head to the cylinder using Permatex Anaerobic gasket maker.
Replace heavy non vented aluminum ball bearing piston head with--
New vented POM piston head to allow lighter piston and head to cycle back more quickly
Lower ratio 13:1 gears - make gun cycle faster
Reuse bevel from KWA
Install SHS plastic sector clip
Remove sector tooth to match short stroked piston
Replace 9mm & 8mm ball bearings with bushings for reliability
Grind down outside of bushing to prevent interferance with selector plate
Remove two loops from tappet spring to make it cycle faster
Cut inspection holes in GB to see the pinion engaging the bevel.
Shim up the new non-self shimming gears and bushings starting with bevel to pinion.
Heavier spring to counter overcycling and premature engagement
New torque motor to pull new speed gears and heavier spring
Rewire with : 3034 FET, 24 amp PTC, 14 AWG wire and terminate with Deans connector
Add hole in back of GB to insert/remove screwdriver with shells closed.

What was not done to the gun
Never removed hop-up/barrel to eliminate possibilities of causing air seal problems too.
reuse KWA ported cylinder, plastic single o-ring cylinder head and o-ring nozzle to preserve good airseal.
11.1V LiPO not used, this gun has been tested only with a 9.6VDC 1600mah NiMH battery.
Re-use spring guide

Parts Used
SRC piston short stroked by one tooth and semi-swiss cheesed. This is a piston I had lying around.
APS POM vented piston head and O-ring- total weight of piston and head is 20 grams
SHS gen2 13:1 speed gear set Spur and Sector - first tooth on the pickup side of sector is ground off
SHS blue plastic sector chip.
KWA bevel gear
Lonesx A2 short, Torque Motor and pinion.
SHS M120 spring
3034 FET with two parallel 12amp PTC protection devices and 14AWG wire and Deans Battery connector.
Reused the motor tabs on new wire.
Scatterplot 3/16th 70D sorbo washer

Efficacy

In the begining this is what the parts looked like before I stripped the piston. This is the pre 2GX gearbox.

Cylinder head before installing the 70d 3/16th piece of sorbo. The rubber was removed from CH and glued onto the sorbo to protect the sorbothane from the piston head. The CH was glued to the Cylinder with Permatex Anaerobic gasket maker. To apply it I pushed the CH in about a quarter of an inch, put the sealer on the CH then pushed it out which prevents the sealer from getting inside of the piston side of the cylinder. After applying the sealer the cylinder was installed in the GB and pushed all the way forward while the sealer set up for 24 hours. Use this stuff and forget about putting teflon on your CH. Teflon is for taking up space in tapered plumbing fittings not your AEG.

This picture shows the geometry of the KWA piston to the gears with the sorbo on the cylinder head. It is hard to see the AOE with the KWA piston This is the piston that stripped.

Next issue was using the right bevel gear with the SHS 13:1 speed gears. Some discussion covered the why here.http://kwausa.com/fo...indpost&p=82665
In the end the KWA bevel was used. With the bushings there was plenty of room to shim the KWA bevel to the pinion.

Inspection holes drilled at bottom of GB so the pinion to bevel interface can be visually set. Other side of the GB has a similar hole drilled. This is the finished gearbox too.

Blow up of the gear in the inspection hole. When shimming the bevel to pinion I measured the distance of the top of the gear to the outside of the GB. This helped assure that the shim used was the right size. Once this shim was in place I put a feeler gage into the other side to find that shim size. There was plenty or room on both sides of the gear to shim up the KWA self shimming gear, the ones with the shims moulded into the gears, using the 5KU bushings.

One last thing. Before snapping the gearbox back together it is important to get the sector timing right. If it is not just right the tappet plate may be pushed up by the sector cam (to advanced) or the cutoff leaver will not be down all the way (too retarded). This picture shows the sector timing just right.

Where do I go from here?

This gun is just a tad hot for me so I would like to reduce the FPS some. Also I would like to be able to run the 11.1V LiPO without fear of stripping the piston. I have a couple of 3 metal tooth SHS pistons coming so I could short stroke it by two once those get here. With the LiPO this gun could get pretty close to a ROF of 30 and then the feeding problems can rear their ugly heads.

Interests:Keeping those damn squirrels from putting their filthy little paws on my peaches in my backyard. Oh, Yes. It's not just a hobby, its a passion!!

Country:

Posted 31 July 2012 - 04:24 AM

Nice report.

With an 11.1V lipo (depending on output rating) and your lonex motor, I would estimate your ROF to be from 32-38 easy. To be absolutely sure of eliminating the possibility of PE I would recommend short stroking by 3-4 teeth. And remember to use a heavier spring to compensate for the loss of FPS from short stroking. I estimate about a 15-20FPS loss for every tooth that is removed. So you may be able to still use the spring that you have or go a step higher.
In addition, at the rate of fire I have mentioned, it maybe necessary to modify your tappet plate along with shortening your tappet spring that will increase tension and allow the nozzle to spring forward faster in order to adjust for timing.

Thanks for the advice. I have already shortened the KWA tappet spring by two links. Do you suppose I need to take out more?

The FET I put in did not originate with me. I did have to open it up and resolder it because one of the wires fell off so I have some pictures. Here are a few pictures.

This one shows the source pin missing the black wire coming from the negative side of the battery. I resoldered the whole thing and re heat shrinked it before use. It is kind of hard to see but there are two of those 12 amp
PTC (yellow) devices in parallel for a 24 amp continuous. It is interesting to see the time-to-open charts of these devices. They can conduct a hundred amps for a few miliseconds so they dont blow on a surge as easily as a fuse.

How I routed the wires.

this is the layout of the device I sent to the guy who constructed it for me.

Here is a pictorial schematic by yet another guy. The 33K could be 1.4 watt. Note that the positive above that goes to the trigger switch is connected to the protected side of the PTC not directly to the battery to prevent an unprotected short of the battery in the trigger circuit.

Shown here is the harness wired to the motor and trigger switch. I was able to re-use the motor connection tabs.

Note the bevel gear inspection hole. That got away from me on the drill press. Gotta use something to hold your work down other than your hand.

Shown here is the harness wired to the motor and the trigger switch. Note that I removed the metal plate from the selector and shorted the "safety" contacts together with a small piece of solid wire.

For me making the stuff is the easy part. Fitting it in there is the trouble. There is a lot of plastic in the KWA G36 around the barrel. SRC and Classic Army have none or little of that. JG , TM and KWA have lots. I removed much of it. I used a dremmel cutoff saw and flush diagonal cutters to chop it out then smoothed it with diamond burr removal bits in the dremmel.

first came figuring where the wires would go.
This stuff gotta go cause it is in the way and no one needs another connection in the routing, especially another Tamiya (amphenol) not to low resistance connector.

This stuff too. I need room for the FET and PTC on one side and someplace to tuck the deans on the other side.

i can't stress this enough put an aluminum heat sink on the fet and bolt it down make sure to put heat sink compound, i can GAURANTEE you it will stay MUCH cooler trust me just do it

I have no argument about heat sinks reducing the temprature of a case,

I do however think it is possible, in the application as a trigger switch, that the case on the 3404 fet is not really that hot.

Why? Because the IR3034 FET is saturated and therefore does not necessarily drop much voltage and therefore is not dissipating much power. Not being one to just blindly follow , I will be testing my next build and measuring the temprature of the case with a non contact thermometer to see how high the temperature gets in a live firing application. I will report back. Hot to my finger is not necessarily hot to a silicon transistor.

If you have an AEG FET that is getting real hot I suggest you check the gate circuit to be sure the voltage divider on it is raising the gate at least to 5-10 VDC to assure total saturation of the drain to source junction. The saturated Vdg should be 0.002 ohms worst case so it would be less most likely.

Some napkin math will reveal that an AEG with a 0.4 ohm motor (30 amps draw with 12 V source) could need to dissipate 1.8 Watt, so there is potential for some real heat. Any motor that runs that low will be blowing fuses right and left so it is probably a high estimate for what a motor would draw too. And motors do not put a continuous load on the battery. What is actually measured though I will report post measurement.

Here is a picture of what a real AEG is asking of a real battery. (scale is 10 amps per division)

none of my fets or my friends fets get hot...i sold them a 7 lead 3034 mosfet that can handle 380 amps soldered it on to a pcb board and added a big 5kw tvs diode to eat any voltage spikes....trust me i haven't had any fets fail on me yet, then i just soak the fet in 100% silicon caulking to waterproof it

i don't like how you soldered on the wire onto the tab you have to be careful when doing that 3034 fets tend to fail easily because of thermal expansion...its not a problem on the 7 lead because it has a drain tab and it is meant to be grounded you should have just bolted it onto the tab not solder :/ i hope your not using that 33k resistor its horrible use a 2.2kohm resistor much more reliable shuts off the fet much quicker and it won't be triggered that easily by any kind of condensation or anything like that also did you change the lonex pinion to the kwa pinion if not your not gonna have proper meshing and its gonna sound terrible don't use the shs bevel depending on which generation it is the bottom drive teeth are made to a specific spec as in the base of it is skinny and upper along the shaft of the teeth it gets wider and it usually fails around there...just use the original kwa beveal and kwa pinion gear

i don't like how you soldered on the wire onto the tab you have to be careful when doing that 3034 fets tend to fail easily because of thermal expansion...its not a problem on the 7 lead because it has a drain tab and it is meant to be grounded you should have just bolted it onto the tab not solder :/ i hope your not using that 33k resistor its horrible use a 2.2kohm resistor much more reliable shuts off the fet much quicker and it won't be triggered that easily by any kind of condensation or anything like that also did you change the lonex pinion to the kwa pinion if not your not gonna have proper meshing and its gonna sound terrible don't use the shs bevel depending on which generation it is the bottom drive teeth are made to a specific spec as in the base of it is skinny and upper along the shaft of the teeth it gets wider and it usually fails around there...just use the original kwa beveal and kwa pinion gear

One of the things I liked about the way that FET was constructed is the way he soldered the wire to the tab. I did my first one last night and did it the same way. The transistor was very clean and took the solder quickly. The completed unit works well.

I used a 273 ohm and 5k ohm on the one I did last night and , as I mentioned above it works well.

I am aware of the tooth problem on the SHS gears. I linked to this in the discussion above, perhaps you missed it.

So as I was packing up to play yesterday, I pulled the KWA off the wall and shot a few over the chrono. Seems the way the spring was parked it liked 380FPS while the hop up was adjusted just right so I put it in the bag and brought it along with my SRC GenIII that I just put a new FET with PTC protection in. It ran beautifully. Shot about 3000 rounds out of it.

Game play and testing I would say I fired it 4000 times since the new piston went in.

Here are some pictures and I am concerned that I did not push the piston back far enough. I could be wrong and perhaps when I knicked the gear while removing the tooth caused this but here they are, pictures of the pickup gear with a little damage.

What do you think?

Here is the new 15 tooth in the gun with 120 spring pushing it and that same tooth on the sector that damaged the plastic piston. Note that the sector teeth are rounded and the piston pickup tooth is flat.

I think I have to go back some more. I am afraid that the metal piston will not be as forgiving as the plastic one.

What do you think?

Is this the error that creates the damage I am seeing on the green piston?

Interests:Keeping those damn squirrels from putting their filthy little paws on my peaches in my backyard. Oh, Yes. It's not just a hobby, its a passion!!

Country:

Posted 07 August 2012 - 04:14 AM

The green piston seems to have signs of wear that may indicate that the engagement tooth on the sector gear is slipping. It may be due to either the 1st tooth on the sector gear not meshing deep enough with the engagement tooth on the piston. This may be from the piston rail channels being cut too wide thus causing the piston to wobble during pickup. One piston you may want to look at is a Azimuth or a Super shooter piston. They fit very well from my experience and is what I use for all my high speed setups. The azimuth can be found on WGCshop.com (1 metal tooth $8.80/ full metal ~$14.00) and the super shooter on ehobbyasia.com (about $18.00). They are the clear blue ones. I have found that both of these pistons are a direct drop in when used in a stock KWA GB without needing AOE adjustments.
The second picture with the metal teeth piston looks like needs about another 2mm sorbo on the cylinder head. And you are correct that they are not forgiving when not tuned correctly for obvious reasons. This is the reason I suggest short stroking from the release side by 2-4 teeth when using full metal teeth since PE would cause catastrophic GB failure.

The green piston seems to have signs of wear that may indicate that the engagement tooth on the sector gear is slipping. It may be due to either the 1st tooth on the sector gear not meshing deep enough with the engagement tooth on the piston. This may be from the piston rail channels being cut too wide thus causing the piston to wobble during pickup. One piston you may want to look at is a Azimuth or a Super shooter piston. They fit very well from my experience and is what I use for all my high speed setups. The azimuth can be found on WGCshop.com (1 metal tooth $8.80/ full metal ~$14.00) and the super shooter on ehobbyasia.com (about $18.00). They are the clear blue ones. I have found that both of these pistons are a direct drop in when used in a stock KWA GB without needing AOE adjustments.
The second picture with the metal teeth piston looks like needs about another 2mm sorbo on the cylinder head. And you are correct that they are not forgiving when not tuned correctly for obvious reasons. This is the reason I suggest short stroking from the release side by 2-4 teeth when using full metal teeth since PE would cause catastrophic GB failure.

Now you have me thinking, a dangerous situation.

Regarding your recommendation on short stroking the sector, are you saying to cut the teeth from the release side of the gear only or a combination of the release and pickup ends? Note I already removed the one tooth from the pickup side. And is the previous advice for both plastic and metal pistons or just the metal pistons only?

Regarding the piston wobble. I was thinking that if I put the piston in the GB and closed it, I would be able to feel the piston wobble, or is it a micro wobble you are talking about.

I tested this easily by putting the original stripped KWA piston back in and trying to feel a difference, but I am wondering if what I can feel will indicate what the piston is feeling. There seems to be little to no difference in how the KWA, green or metal piston feels to my finger in the GB.

Not a huge difference in measurements, the aftermarket ones just under 1mm narrower, but I wonder what part hits the piston, the outside rail of the piston hitting the side walls of the GB or the piston rail in the GB hitting the sidewall of the piston inside of the pistons two grooves?

Interests:Keeping those damn squirrels from putting their filthy little paws on my peaches in my backyard. Oh, Yes. It's not just a hobby, its a passion!!

Country:

Posted 07 August 2012 - 05:37 AM

I would short stroke from the release side on the sector gear and same number of teeth have to be taken off the piston unless you are working with full metal or half metal rack.
I did see that you took one tooth off the pickup side and I don't know how the timing will work out in your situation when more teeth are taken off the release side.

And the width of the channels doesn't seem to be a big difference so lets look at the overall diameter of the pistons and the height of the engagement tooth on the pistons. See if you can see a difference in height from the tip of the 1st tooth on the piston down to the outer edge of the opposite side. I am thinking that maybe the the green piston is skinnier. Just a thought.

I recall you cut part of a GB out so you could see AOE. Is that a test GB or the one you are using? I am thinking I want to cut some of the KWA GB away like that so I can see the AOE with the spring in and the GB closed. Now I can only see it with the GB open and the piston is not in it's proper place.

Here is the difference between the scored SHS green piston and the unknown brand purple 15 tooth.

It was pretty hard for me to get this photo to show what the eye sees but the metal tooth appears to have a steeper angle than the plastic one. The angle of the metal tooth is more like what the sector carved into the green. Maybe that is better or worse I don't know other than they appear different. I cant tell the KWA because of the shield on either side and of course the pickup tooth was removed by the sector on that one.

As far as adding 2mm or so to the AOE I can switch out the 3/16 sorbo for a 1/4" one which is about 2mm thicker. The rubber I put over it is what was originally on the KWA CH.

It was a test gearbox shell but I don't see why it can't be done to a working gearbox. Just be sure that the interior surface is free of burrs.

Taking a second look the side of the piston that does not have reinforcement is on the side of the GB where the tappet rail is. I am not sure it would be a good idea to cut the tappet rail there, where the sector cam wants to push it up. Maybe if I cut around it but I need to be more sure first. In the last picture above there is that round protrusion with the smiley below it. On the G36 does that have any function?

Interests:Keeping those damn squirrels from putting their filthy little paws on my peaches in my backyard. Oh, Yes. It's not just a hobby, its a passion!!

Country:

Posted 07 August 2012 - 08:26 AM

The circular protrusion and the "smiley" (lol) is where selector gear would attach to if the gun was an AK, sig551-556 and etc.
And now that I think of it, you may be right that it would cut into the tappet rail and can cause failure. Thanks for pointing that out.
Maybe you can just drill a hole small enough that it will not cut into the tappet rail.
Thanks and you are very welcome.
Now let's see if we can solve your mystery.

I have a spare gearbox that needs some 6mm bushings so I was thinking about making that a test GB. I tried to put one side of this SRC GB with the KWA and it will not click together because the KWA has some little extras on the two sides at the bottom side of the front under the CH that prevent it from snapping together with the SRC GB.
So it gets me to wondering, If I mod the SRC GB and view the AOE through the hole will that be the same when I put the guts into the KWA GB which gets me back to the compatibility issue that was becoming a non issue to me.